A User Equipment (UE), also known as a mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between UEs, between a UE and a wire connected telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The UE may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The UE in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another UE or a server.
The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS) or Base Transceiver Station (BTS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and/or terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on radio frequencies with any UE within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the UE. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node.
Beyond 3G mobile communication systems, such as e.g., 3GPP LTE, offer high data rate in the downlink by employing multiple antenna systems utilising Multiple-Input and Multiple-Output (MIMO).
Massive MIMO is a recently emerged technology that uses large Antenna Arrays Systems (AAS) with individual transceivers to improve throughput of wireless communication systems. Massive MIMO may sometimes also be referred to as “very large MIMO system”, or “large-scale antenna system”.
Antenna arrays with large number of elements enable the increase in capacity by utilising spatial beam forming and spatial multiplexing. The benefit of these large arrays is the ability to spatially resolve and separate received and transmitted signals with very high resolution.
The resolution is determined by the number of antenna elements, and their spacing. Typically the number of transceivers may be as high as 10× the maximum rank of the system. The rank is defined as the total number of parallel (same time and frequency) transmissions, including both wanted and unwanted signals (i.e. interference). Massive MIMO is sometimes loosely defined as a system using comprising 100 or more transceivers.
Basically, the more antennas the transmitter/receiver is equipped with in massive MIMO, the more the possible signal paths, the better the performance in terms of data rate and link reliability.
Advantages with massive MIMO comprise improved UE detection. Further, thanks to the high resolution of massive MIMO, the transmit power per UE may be reduced.
Both single user MIMO with many layers and multi user MIMO will increase the network performance and system capacity. Especially in the uplink, the radio network node will have the new freedom of spatial diversity to handle interference and increase the Signal Interference Noise Ratio (SINR). One major challenge to implement Massive MIMO technology will be that the number of antenna streams will increase with the number of elements. The many elements result in a significant increase in complexity, which has to be handled by the Hardware (HW) and Software (SW) architectures. For the user scheduling, the spatial domain exposes new parameters to be used in the many element AAS and increases the complexity even further for the user scheduling algorithms.
Further, accurate channel state information must be acquired to reap the benefits of additional antennas in massive MIMO. This poses, in particular in fast fading channels, a challenge as the number of antennas grows.
The complexity of baseband receive and transmit MIMO algorithms scales exponentially with number of antennas, leading to high requirements for computational ability, which may require additional dedicated hardware in form of very high capacity processing platforms for implementing massive MIMO. Further, computational complexity adds processing time, delaying the transmission/reception, and consume power, leading to high energy costs and additional heating.
Thus an implementation of the increased number of antenna elements would not be realistic due to the incremental increasing resources needed for base band processing; also the interface requirements would be out of scope.
It appears that massive MIMO requires further development for becoming feasible for practical implementation.